In a mobile communication system represented by the LTE (Long Term Evolution) scheme including only a “Packet-switched network (PS Network)”, it is common to implement voice communication via the VoIP (Voice over IP) that transmits and receives packets loaded with voice codec.
However, in a mobile communication system of the UMTS (Universal Mobile Telecommunications System) scheme having an existing circuit-switched network (CS Network), the provision of voice communication via the VoIP has many technical hurdles in view of communication continuity (handover process, etc.) with the usual voice communication.
Further, if a service is provided only through a circuit-switched network, the service may become “unusable” in a packet-switched network.
Therefore, in order to perform circuit-switched communication such as voice communication in a packet-switched network, there is known a method in which a visitor is moved to a mobile communication system having a circuit-switched network, and then circuit-switched communication is performed.
The method is called “CS Fallback (CSFB)” when executed between a mobile communication system of the LTE scheme and a mobile communication system of the UMTS scheme, between a mobile communication system of the LTE scheme and a mobile communication system of the GSM scheme, or else between a mobile communication system of the LTE scheme and a mobile communication system of the CDMA 2000 scheme.
The operation executed in the case of a mobile station location-registered on a mobile communication system of a first communication scheme (for example, the LTE scheme) including only a packet-switched network, and then falling back to a mobile communication system of a second communication scheme (for example, the UMTS scheme, the GSM scheme, or the CDMA 2000 scheme) having a circuit-switched network to start the circuit-switched communication is as follows.
Firstly, in a mobile station UE, a NAS protocol function (for example, EMM) of the first communication scheme transmits a “circuit-switched outgoing call request signal (Service Request)”, to an AS protocol function (for example, LTE-AS) of the first communication scheme.
Secondly, in the mobile station UE, the AS protocol function of the first communication scheme transmits a “Fallback request signal (Service Request)” for the circuit-switched network, to the packet-switched network.
Thirdly, the packet-switched network transmits the “Handover instruction” or the “RAT change request” for the circuit-switched network, to the mobile station.
Fourthly, the mobile station transmits the outgoing call signal for circuit-switched communication, to the circuit-switched network.